1. Field of the Invention
The present invention relates to spread spectrum (SS) communication systems using Pseudo-Noise (PN) coding techniques and, more particularly, to generating PN code phases in code division communications systems.
2. Prior Art
Spread spectrum (SS) systems, which may be Code Division Multiple Access (CDMA) systems, are well known in the art. SS systems can employ a transmission technique in which a pseudo-noise (PN) PN-code is used as a modulating waveform to spread the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver the signal is de-spread using a synchronized replica of the PN-code.
In general, there are two basic types of SS systems: direct sequence spread spectrum systems (DSSS) and frequency hop spread spectrum systems (FHSS).
The DSSS systems spread the signal over a bandwidth fRF±Rc, where fRF represents the carrier frequency and Rc represents the PN-code chip rate, which in turn may be an integer multiple of the symbol rate Rs. Multiple access systems employ DSSS techniques when transmitting multiple channels over the same frequency bandwidth to multiple receivers, each receiver sharing a common PN code or having its own designated PN-code. Although each receiver receives the entire frequency bandwidth, only the signal with the receiver's matching PN-code will appear intelligible; the rest appears as noise that is easily filtered. These systems are well known in the art and will not be discussed further.
FHSS systems employ a PN-code sequence generated at the modulator that is used in conjunction with an m-ary frequency shift keying (FSK) modulation to shift the carrier frequency fRF at a hopping rate Rh. A FHSS system divides the available bandwidth into N channels and hops between these channels according to the PN-code sequence. At each frequency hop time a PN generator feeds a frequency synthesizer a sequence of n chips that dictates one of 2n frequency positions. The receiver follows the same frequency hop pattern. FHSS systems are also well known in the art and need not be discussed further.
In general, although the original data stream is recovered, after PN acquisition, the actual data cannot be recovered, or extracted from the data stream until data-symbol boundaries are identified. Data-symbol boundaries are identified either with a symbol synchronizer (bit synchronizer, with its attendant acquisition and pull-in time), or with PN code epochs (i.e., PN code phase).
A DSSS communication element requires its locally generated PN code to substantially match the intended, or received, composite code phase as indicated by its local counters and registers. Thus, a Direct Sequence Spread Spectrum communication element needs to know if its local version of the PN code is at the same phase as the rest of the network. As networks become more complex, and low latency becomes more important, less time must be spent in acquiring lost PN sequence timing. The first advantage of this method is that it produces a set of long codes with favorable autocorrelation properties. The MOD version of these composite codes have no autocorrelation, except where they perfectly align. Another advantage is that many functionally independent codes can be generated with the same set of hardware.
The main advantage of this method for producing multiple codes is that any user can switch to a new code while maintaining lock on all sequences. Since the same set of hardware is producing all the codes, the user in the network synchronized to one code is synchronized to every code. By switching to another code phase combination, the user can transmit or receive any code in the network instantly and then switch back to any other code phase.